Rewrite bear PNGs to be stored more effective

Save 25%/35% of the PNG size by just rewriting the pngs with stipped unneeded metadata and an optimized color palette.

Before:

```
17194 assets/img/glowing_bear_128x128.png
95699 assets/img/glowing-bear.png
```

After:

```
12835 assets/img/glowing_bear_128x128.png
61297 assets/img/glowing-bear.png
```